A steering apparatus shown in FIG. 11, for example, has been conventionally used as an automobile steering apparatus. The steering apparatus is configured to convert rotational motion of a steering wheel 1 operated by a driver into linear motion at a steering gear unit 2, thereby applying a desired steering angle to left and right steering wheels (not shown). In order to implement this configuration, the steering wheel 1 is fixed to a rear end portion of a steering shaft 3. Also, a front end portion of the steering shaft 3 is coupled to a base end portion of an input shaft 6 of the steering gear unit 2 through a pair of universal joints 4a, 4b and an intermediate shaft 5. Also, a pair of tie rods 7, 7, which configure the steering gear unit 2 and are respectively coupled to the left and right steering wheels, are coupled at base end portions thereof to both end portions of a rack shaft, which has rack teeth meshed with pinion teeth provided at an intermediate part of the input shaft 6, through spherical joints.
In the steering apparatus configured as described above, when the driver operates the steering wheel 1, the rotation of the steering wheel 1 is transmitted to the input shaft 6 through the steering shaft 3, the universal joints 4a, 4b and the intermediate shaft 5. As a result, the rack shaft is displaced in a shaft direction (a width direction of a vehicle body) and the tie rods 7, 7 are thus pushed or pulled, so that the desired steering angle is applied to the left and right steering wheels.
The intermediate shaft 5 configuring the steering apparatus as described above is required to have not only a function of transmitting the torque between the universal joints 4a, 4b but also a function of expanding and contracting an entire length thereof due to one or both of following reasons (1) and (2).
(1)For the purpose of smoothly adjusting a position of the steering wheel 1 in a front-rear direction.
An interval between the universal joints 4a, 4b may be varied depending on position adjustment of the steering wheel 1 in a front-rear direction. In this case, in order to smoothly perform the position adjustment in the front-rear direction, it is necessary to configure the intermediate shaft 5 so that an entire length thereof can be smoothly (with small force) expanded and contracted.
(2) For the purpose of protecting a body of the driver at collision accident.
At collision accident, as an automobile primarily collides with another automobile and the like, the steering gear unit 2 is strongly pushed rearwards. If the movement of the steering gear unit 2 is transmitted to the steering shaft 3, as it is, the steering wheel 1 supported and fixed to the rear end portion of the steering shaft 3 is furiously displaced (pushed up) towards the body of the driver, so that it is difficult to protect the driver. Therefore, the intermediate shaft 5 is configured so that the entire length is contracted by shock load resulting from the primary collision. Thereby, the movement of the steering gear unit 2 resulting from the primary collision is not transmitted, as it is, to the steering shaft 3. Incidentally, in order to achieve this function, the load necessary to expand and contract the intermediate shaft 5 may be large to some extent.
The intermediate shaft 5 configuring the steering apparatus is required to have the function of expanding and contracting the entire length thereof, in addition to the function of transmitting the torque, as described above. The intermediate shaft 5 having those functions has been conventionally widely used (for example, refer to Patent Documents 1 and 2). FIGS. 12, 13A and 13B illustrate an example of the intermediate shaft 5 conventionally widely used. The intermediate shaft 5 has a pair of torque transmission shafts 8a, 8b having a round rod shape or round tube shape. Base end portions of yokes 9, 9 for configuring universal joints 4a, 4b are respectively coupled and fixed to axially outer end portions (base end portions at opposite sides) of the torque transmission shafts 8a, 8b. The universal joints 4a, 4b are respectively configured to couple other yokes 9a, 9b to tip portions of the yokes 9, 9 through cross shafts 10 so that the torque can be transmitted between the respective yokes 9, 9a, 9b even at a state where central axes of the yokes 9, 9a, 9b are inclined relative to each other.
A tip portion (a left end portion in FIG. 12) of the torque transmission shaft 8a having the round rod shape of the torque transmission shafts 8a, 8b configuring the intermediate shaft 5 is made to have a larger diameter than an intermediate part thereof and male spline teeth are formed on an outer periphery of the large diameter part, so that a male spline part 11 is provided on the outer periphery of the tip portion of the torque transmission shaft 8a. In this conventional structure, a groove bottom diameter d11 of the male spline part 11 is made to be larger than an outer diameter Dga of the intermediate part of the torque transmission shaft 8a (d11>D8a), so that the large diameter part can be formed with the male spline teeth by broach processing, which is a type of cutting processing. In contrast, an inner periphery of the torque transmission shaft 8b having the round tube shape is formed with female spline teeth over a substantial entire length of the inner periphery of the torque transmission shaft 8b, so that the substantial entire inner periphery of the torque transmission shaft 8b is formed as a female spline part 18.
As shown in FIG. 12, the male spline part 11 formed on the outer periphery of the tip portion of the torque transmission shaft 8a having the round rod shape is engaged with the female spline part 18 provided on the inner periphery of the torque transmission shaft 8b having the round tube shape, so that it is possible to obtain the intermediate shaft 5, of which an entire length can be expanded and contracted by the torque transmission shaft 8a and the torque transmission shaft 8b. In this case, when the spline parts 11, 18 are axially displaced by small force, it is possible to obtain the functions corresponding to the above (1) and (2). Likewise, when the spline parts are axially displaced by larger force to some extent, it is possible to obtain only the function corresponding to the above (2) (as the intermediate shaft 5 is contracted, the shock energy input from the front end portion of the intermediate shaft 5 upon the collision accident can be absorbed).
Regarding the respective members of the intermediate shaft 5 having the universal joints 4a, 4b provided at both end portions thereof, when the torque transmission shaft 8a having the round rod shape and the yoke 9 are integrally formed to obtain a torque transmission shaft having a universal joint yoke, it is not necessary to perform an operation of coupling and fixing the torque transmission shaft 8a and the yoke 9 by welding and like and it is possible to increase the strength of the coupling part, which are advantageous to the saving of the cost of the intermediate shaft 5 and the improvement on the strength thereof. FIGS. 14A to 14C illustrate an example of a conventional method of manufacturing a torque transmission shaft having a universal joint yoke, in which the torque transmission shaft 8a having the round rod shape and the yoke 9 are integrally formed.
According to the conventional manufacturing method, plastic working such as forge processing is performed for a hollow or solid metal material such as medium-carbon steel, so that a first intermediate material 12 as shown in FIG. 14A is obtained. One axial half part (a part except for the other axial end portion, a left part of FIGS. 14A to 14C) of the first intermediate material 12 is formed as a first base rod part 13 having an outer diameter D13 which is constant over a substantial entire length, and a base end portion of the yoke 9 is fixed to the other axial end portion (a right end portion in FIGS.
14A to 14C) of the first base rod part 13, so that the first base rod part 13 and the yoke 9 are integrally formed. The outer diameter D13 of the first base rod part 13 is larger than the outer diameter Dga of the intermediate part (the part axially distant from the male spline part 11 and the yoke 9) of the completed torque transmission shaft 8a having the round rod shape shown in FIGS. 12, 13A and 13B and is substantially the same as a tooth tip circle diameter (a circumscribed circle diameter) D11 of the male spline part 11 (D11 ≈D13>D8a). The cutting processing such as turning machining is performed for the other axial end portion (an axially intermediate part of the first intermediate material 12) of the first base rod part 13, so that a second intermediate material 14 as shown in FIG. 14B is obtained. In the second intermediate material 14, an outer diameter D15 of the other axial end portion (an axially intermediate part of the second intermediate material 14) of a second base rod part 15 provided at one axial half part thereof is made substantially the same as the outer diameter D8a (D15≈D8a) of the intermediate part of the completed torque transmission shaft 8a having the round rod shape and one axial end portion is provided with a large diameter part 16 having an outer diameter which is larger than the other axial end portion. Then, an outer periphery of the larger diameter part 16 is formed with male spline teeth, so that the outer periphery of the corresponding part is provided with the male spline part 11. Thereby, a torque transmission shaft 17 having a universal joint yoke, in which the torque transmission shaft 8a having the round rod shape and the yoke 9 are integrally formed, is obtained.
According to the torque transmission shaft 17 having the universal joint yoke, it is not necessary to perform an operation of coupling and fixing the torque transmission shaft 8a and the yoke 9 by the welding and like and it is possible to increase the strength of the coupling part, which are advantageous to the saving of the cost of the intermediate shaft 5 and the improvement on the strength thereof, like the conventional structure shown in FIG. 12. However, it is necessary to perform the cutting processing such as turning machining for the other axial end portion of the first base rod part 13, so that the manufacturing cost of the torque transmission shaft 17 having the universal joint yoke is increased. Also, the groove bottom diameter d11 of the male spline part 11 is increased beyond necessity, so that it is difficult to reduce a weight of the entire intermediate shaft 5 including the torque transmission shaft 17 having the universal joint yoke.